A sapphire single crystal is a material excellent in transmittance and mechanical characteristics. For example, the sapphire single crystal is in widespread use as an optical material, and in increasing use as an epitaxial substrate for GaN growth.
The sapphire single crystal has heretofore been obtained by being grown from a seed crystal using a crucible made of iridium, tungsten, or molybdenum and using the pulling method (also called the Czochralski method, the CZ method, or the like), the edge-defined film-fed growth (EFG) method, or the Kyropoulos method.
Of those, molybdenum is inexpensive as compared to iridium and tungsten, and hence has been widely used as a material for the crucible.
Meanwhile, sapphire has a melting point of more than 2,000° C. Accordingly, when molybdenum is used in the crucible, it is necessary to improve heat resistance of the crucible.
As a technology for improving the heat resistance of the crucible in which molybdenum is used, there is known a technology involving coating an outer periphery of molybdenum with a film of tungsten or the like having a higher melting point than that of molybdenum (Patent Document 1).
In addition, there is also known a technology involving doping molybdenum with elemental lanthanum or lanthanum oxide (Patent Document 2).
Further, there is also known a technology involving coating molybdenum with tungsten and then subjecting the resultant to heating treatment to form a solid solution layer of tungsten and molybdenum (Patent Document 3).
In addition, there is also known a technology involving forming a metal foil, which does not react with a melt and has a melting point of 1,800° C. or more, on the molybdenum crucible in a separable manner (Patent Document 4).
Meanwhile, in recent years, the sapphire single crystal has been increased in size for an improvement in yield of sapphire, resulting in emergence of a sapphire single crystal having such a size that the sapphire single crystal is difficult to grow by the related-art sapphire single crystal production method such as the pulling method described above.
Under such circumstances, a heat exchange method (HEM) has found use as a growth method applicable to such increase in size of the sapphire single crystal (Non Patent Document 1).